1. Field of the Invention
The present invention relates to an ultrasonic motor (wave driven motor), in which travelling waves are generated in an elastic body to convert wave energy thereof into a thrust or into a torque. More particularly, the invention relates to a travelling wave type motor having a long life with little reduction in performance by an arrangement to make generation of standing waves spatially changeable, which are sources of travelling waves.
2. Related Background Art
A typical example of conventional travelling wave type ultrasonic motors is an annular motor as described in "Introduction to Ultrasonic Motors" (Sougo Denshi Shuppan, 1991). The annular motor is structured as follows. A vibrator is formed such that piezo-electric elements, which are electro-mechanical energy conversion elements, are adhered to a back face of an annular elastic body to form travelling waves on a surface of the elastic body. A moved body pressure-contacts a travelling wave forming plane of the vibrator to be friction-driven by the travelling waves. There is a proposal of such a reverse arrangement with necessity that the moved body is fixed and the vibrator moves. FIG. 2 is a drawing to show an arrangement of piezo-electric elements in the vibrator of the annular motor. There are provided two phases of electrode groups for drive, which will be hereinafter referred to as A phase and B phase with a spatial phase difference of 90 degrees in the vibrator on the right side and on the left side in FIG. 2. In each of the A and B phases, the piezo-electric element plates having alternating polarity, as shown. A pair of positive and negative piezo-electric plates in each phase correspond to one wave length. Respective voltages different in time phase by 90 degrees are applied to the A and B phases. A travelling wave is generated as a result of synthesis of a standing wave generated by the A phase with a standing wave generated by the B phase.
In the above-described conventional technique, a standing wave for each of the phases is generated at a constant or fixed position of the vibrator. Therefore, if the two phases have respective standing waves with a spatial phase difference and a time phase difference both of 90 degrees and with an equal amplitude, an ideal travelling wave would be formed. It is, however, difficult to satisfy such ideal conditions actually.
If the conditions for generating a travelling wave are incomplete, then a wave thus generated is a superposition of a travelling wave having a main component, other travelling wave components different in frequency and standing wave components. The standing wave components are fixed relative to the arrangement of piezo-electric elements, so that the moved body is frequently hit at friction faces located at antinodes of the standing waves. Entended operation of the travelling wave type motor with such an arrangement thus results in abrasion at fixed positions of the vibrator, that is, at the locations of antinodes. This reduces a surface evenness of a working face causing a dispersion of flatness. Vibrations obtained from ordinary piezo-electric bodies and elastic bodies industrially used have amplitudes on an order of at most several microns. The dispersion of flatness on a working face causes unevenness of torque or of thrust, which is a main factor of decrease in performance of an actuator. This in turn results in a lowering of durability.